1. Field of the Invention
This invention relates to a catalyst for purification of exhaust gases in internal combustion engines, etc. More specifically, this invention relates to a catalyst suited for purification of nitrogen oxides exhausted from internal combustion engines.
2. Description of the Prior Art
Removal of atmospheric pollutants is a serious problem from the aspect of global environment conservation. Above all, removal of nitrogen oxides which are said to cause photochemical smog or acid rain is an urgent problem.
A selective catalytic reduction method using ammonia has been so far applied to exhaust gases from large-scaled fixed generation sources such as a thermoelectric power plant, etc., and some results have been thereby accomplished.
Meanwhile, a three way catalyst (TWC) method in which an air/fuel ratio is controlled to about a stoichiometric amount (A/F=14.7) and carbon monoxide (CO), hydrocarbons (H.C.) and nitrogen oxides (NOx) are simultaneously removed has been applied to exhaust gases of engines for on-site cogeneration or internal combustion engines of automobiles, trucks, etc.
However, in recent years, the amount of carbon dioxide (CO.sub.2) exhausted has to be suppressed for prevention of global warming, and it has been demanded to put a lean burn engine to practical use. Nevertheless, the three way catalyst is not effective therefor.
Although a Diesel engine is inherently a lean burn engine, there has been no alternative but to use means such as exhaust gas recycle (EGR), fuel injection timing retard, etc. in order to suppress NOx of exhaust gases, and occurrence of floating particulates has been permitted to some extent. However, strict exhaust regulation will be imposed on both the particulates and NOx in the future. By the way, it is deemed that occurrences of particulates and NOx are in trade-off relationship in exhaustion of the Diesel engine and if the catalyst can effectively remove NOx, occurrence of particulates can be greatly suppressed.
Hereinafter, lean burn-system engines such as a lean burn gasoline engine, a Diesel engine and a gas engine are generally termed lean burn engines.
In order to purify nitrogen oxides in exhaust gases of the lean burn engines containing oxygen in excess of the stoichiometric amount, zeolite catalysts such as copper ion exchanged zeolites (U.S. Pat. No. 4,297,328 and Japanese Laid-open Patent Application No. 100919/1988), ion exchanged zeolites by noble metals such as platinum, palladium, rhodium, etc. (Japanese Laid-open Patent Application No. 135541/1989), and the like have been proposed in recent years.
These conventional zeolite catalysts nevertheless suffer fatal defects that the effective temperature region of NOx purification is limited and activity is heavily degraded in high-temperature exhaust gases containing steam. Thus, their performance has not yet reached the practical level. It has been generally presumed that low heat resistance of the zeolite catalysts is attributed to thermal instability of a zeolite crystal structure.